Ricin is a toxic protein isolated from Ricinus communis (castor bean) which exerts its activity only on eucaryotic ribosomes. We have devised a new affinity column isolation procedure to obtain highly purified material, both ricin and the nontoxic agglutinin found in castor beans. We have studied the amino terminal sequences of the A and B chains of ricin and also of the agglutinin. Sequence data indicates that the A chains are homologous polypeptides. The A chain from both ricin and the agglutinin are effective in preventing protein synthesis in vitro. A probable sequence to which carbohydrate is covalently attached has been identified. We have studied the binding of ricin and the isolated A and B chains to ribosomes. We have found that all three bind to ribosomes, but with different stoichiometry. We have shown that ricin does not act as an endonuclease by using P32 labelled polysomes from L cells. No cleavage of 28s, 18s and 5s ribosomal RNA could be found. Ricin did not have any effect on polysomal profile on sucrose density gradient centrifugation. This indicates that it does not cleave mRNA. Work is underway to identify the site of binding within the ribosome. BIBLIOGRAPHIC REFERENCES: Mitchell, S., Hedblom, M., Cauley, D., and Houston, L. L. "Ricin Does Not Act as an Endonuclease on L Cell Polysomal RNA", Bichem. Biophys. Res. Commun. In press, 1976.